netdev - Re: sys_paccept: disable paccept() until API design is resolved

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Date:	Tue, 16 Sep 2008 17:04:38 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Michael Kerrisk <mtk.manpages@...glemail.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Davide Libenzi <davidel@...ilserver.org>,
	Alan Cox <alan@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Jakub Jelinek <jakub@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	netdev <netdev@...r.kernel.org>,
	Roland McGrath <roland@...hat.com>,
	Christoph Hellwig <hch@....de>
Subject: Re: sys_paccept: disable paccept() until API design is resolved

On 09/16, Michael Kerrisk wrote:
>
> * The behavior of paccept() when interrupted by a signal is IMO
>   strange: the kernel restarts the system call if SA_RESTART was set
>   for the handler.  I think that it should not do this -- that it
>   should behave consistently with paccept()/ppoll()/epoll_pwait(),
>   which never restart, regardless of SA_RESTART.  The reasoning here
>   is that the very purpose of paccept() is to wait for a connection
>   or a signal, and that restarting in the latter case is probably
>   never useful.  (Note: Roland disagrees on this point, believing
>   that rather paccept() should be consistent with accept() in its
>   behavior wrt EINTR
>   (http://thread.gmane.org/gmane.linux.kernel/723953/focus=732255).)

Also, the implementation of sys_paccept() is not "perfect", imho.

	sys_paccept:

		ret = do_accept(...);

		if (ret < 0 && signal_pending()) {
			set_restore_sigmask();
			return ret;
		}

It doesn't check that ret == ERESTARTSYS/EINTR. I can't say this
is bug, but let's suppose that do_accept() returns (say) -EINVAL,
and then the task is interrupted by the signal.

Now, if the signal comes after sys_paccept() checks signal_pending(),
we return -EINVAL, and the signal handler runs with the original
current->blocked mask, as expected.

However, if the signal happens in the window before signal_pending(),
we still return -EINVAL, but the signal handler runs with
->blocked == sigmask. A bit odd, but probably harmless.


Note also that unless I misread the code, do_paccept() returns
ERESTARTSYS or EINTR depending on ->sk_rcvtimeo. Yes, it is very
clear why sock_intr_errno() does this, but this doesn't make the
behaviour of paccept() more understandable.

Oleg.

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